1. Field of the Invention
The present invention relates to an image processing method and apparatus for processing an image as a digital signal and, more particularly, to an image processing method and apparatus for reproducing an image of a high picture quality.
2. Related Background Art
In general, digital copying apparatuses in which an image is sampled by a CCD sensor or the like and digitized data is output from a digital printer such as a laser beam printer or the like thereby reproduce an image are widespread in place of conventional analog copying apparatuses, due to the development of digital apparatuses.
The digital copying apparatus generally uses a method of reproducing gradations by a dither method or a concentration pattern method in order to reproduce halftone. However, such a method has the following drawbacks.
(1) In the case where an original is a dotted image which is formed by printing or the like, a periodic fringe pattern which does not exist on the original appears in the copied image. PA1 (2) When diagrams, characters, or the like are included on an original, the edges become uneven due to the dither process, so that the picture quality deteriorates. PA1 (3) And the like. PA1 (A) Beats between the dotted original and the input sampling. PA1 (B) Beats between the dotted original and the dither threshold value matrix.
The phenomenon (1) is called a moire phenomenon and it is considered that such a phenomenon occurs by the following causes.
Particularly, in the phenomenon (B), in general, when the threshold values of the dither are arranged as a dot concentration type, an output image also has a pseudo-dotted; structure. Such a dotted structure causes beats with the input dotted original, so that the moire phenomenon occurs.
On the other hand, there has been known an error diffusion method as a binarizing method which has recently been highlighted. According to this method, a concentration difference between an image concentration of an original and an output image concentration for every pixel is calculated and the error amounts resulting from the calculations are diffused by being added, after weighting, to the peripheral pixels. Such a method has been published in the literature by R. W. Floyd and L. Steinberg, "An Adaptive Algorithm for Spatial Grey Scale", SID. 75, Digest.
On the other hand, although there has also been known a method called a least mean error method, such method is considered to be substantially equivalent to the error diffusion method.
According to the error diffusion method, since no periodicity exists in the binarizing process, no moire occurs for the dotted image, so that there is an advantage that the resolution is high as compared with that of the dither method or the like. However, in the case of processing uniform concentration portions (highlight portion, shadow portion) of an image, a unique fringe pattern occurs, so that there is a drawback that the picture quality deteriorates.
To eliminate the above drawbacks, U.S. Patent Nos. 4,878,125 and 4,876,610 and U.S. patent applications Ser. Nos. 145,593 and 192,601 have already been filed on behalf of the assignee of the present invention.
Among them, according to the U.S. patent application Ser. No. 192,601, after a dotted image is formed in the highlight portion of the image, the binarizing process is executed by the error diffusion method, thereby preventing the generation of a fringe pattern or particle-shaped noises when processing by the error diffusion method.